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Partner in solar power

19 July 2011George Marsh

Part one: Solar PV modules would be of little use was it not for an electronic box of tricks that is normally out of sight and out of mind - the inverter.

The central role of the inverter is to convert the direct current delivered by the solar panels to an alternating current that can be used by electrical appliances or fed to the grid. Why, then is the device not simply called a converter? Because, in the past, that was what electrical engineers called rotary motor-generator machines that converted ac to dc, and operating this process in reverse to change dc to ac was the inverse of the earlier use.

Hence “inverter”. Rotary conversion has long been superseded, first by electromechanical switches, then by vacuum tubes and latterly by semiconductor switches - silicon controlled rectifiers, power transistors etc. Switching is used to repeatedly reverse the polarity of the input dc to produce a square wave, which can then be conditioned to form a sine wave.

A sine wave of only modest quality may be compatible with much electrical equipment, but for more sensitive items and certainly for input to the main power supply grid, a more ideal sine wave is needed. Grid-tie solar inverters require circuitry to make the output fit within tightly specified limits for voltage, frequency, phase, harmonic distortion and power factor. Moreover, safeguards are needed to prevent dc from being fed into the grid, to avoid ‘islanding’ (with battery back-up inverters) and to ensure automatic shut-down in the event of a grid outage (or fault ride-through for more advanced types). Means to control the grid connection/disconnection process must also be present. All this makes for a sophisticated device.

A sine wave of only modest quality may be compatible with much electrical equipment, but for more sensitive items and certainly for input to the main power supply grid, a more ideal sine wave is needed.

Another layer of complexity comes with the ability to maximise power extracted from the solar array using a maximum power point tracking (MPPT) technique and, increasingly, to provide remote array monitoring and data logging. Yet inverters must be as small and light as possible, highly efficient (upwards of 94% conversion efficiency), reliable and durable. Grid-connected types must be tested and approved to the rigorous standards set by the utility grid authorities. Whilst providing this shopping list of qualities, these electronic miracle workers must also be affordable.

Trends

Solar inverter designers seek constantly to reduce the form factor (space taken up) and weight of their devices whilst increasing efficiency, grid compatibility and the effectiveness of features such as MPPT, control of battery charging and switchover where battery energy storage is used, and communications with the outside world. One strong drive is to reduce component count, so as to increase reliability and lower the price.

This has resulted in the adoption of integrated circuits and, in line with the digital revolution, digital processing ‘chips’. In turn, this has opened the door to software content, a move that has made inverters more versatile since functions can be set and changed in software without involving the hardware. Indeed, today's ‘smart’ solar inverters can be regarded as largely digital microcontrollers which, in addition to the basic inverter role, can execute many related functions.

In time, module makers may start to produce AC modules that combine micro-inverters and panels in single easy-to-install ‘plug and play’ products.

The rapid switching capability of power transistors makes it possible to alter, or “modulate” the square wave output from the initial.. …inverter stage by switching the output on and off a number of times during each pulse. As a result the effective width of the pulse and its energy content are changed. This process of pulse width modulation can be used to control output parameters ranging from voltage to harmonic distortion. PWM is a powerful means of control and designers are constantly refining their modulation algorithms.

There are technological areas where trends are not yet fully clear. One of these concerns the use of transformers. Conventional transformers are used to transfer ac from inverters to the grid inductively, without direct wired connection, so that there is isolation in the event of a fault. However, low frequency transformers are bulky and “lossy”, having an adverse effect on conversion efficiency so, where network regulations permit, many designers try to do without them. Transformerless inverters, with their reduced weight and higher efficiencies, have become popular in Europe, despite residual concerns over the possibility of transmitting dangerous dc faults to the ac side and the grid.

A now popular alternative is to retain the isolating advantages of a transformer, but use one that operates at high ac frequency. This is because electrical losses are much reduced at higher frequencies. Even allowing for the necessary multi-step conversion process - from dc to high-frequency ac, then back to dc and thence to the final ac output at the target frequency and voltage - this can still provide an overall gain in efficiency. Newer compact transformers for solar inverters use a multi-step computerised process to carry out the conversions. Unfortunately, the additional electronics required for HF operation add to overall expense, so designers must calculate the trade-offs in making their transformer decisions.

The micro alternative

A current debate in the inverter world concerns whether all inverter functions for a solar installation should be located in a single central cabinet or distributed throughout the array by locating a micro-inverter with each solar panel. Traditional practice has been to connect panels in series, the resulting high dc voltage being fed to a central inverter. For larger installations, the total array is divided into a number of series-connected array ‘strings‘, the dc output from each of which is passed to a string inverter.

A radical departure that has become possible with the continued shrinkage of electronics is to locate a small, micro, inverter with each panel and connect all their ac outputs in parallel and then to the grid. Distributed micro-inverter solutions have been gaining traction, especially in the U.S.

Q&A: what the experts think – Fronius

What are the main design and usage trends in inverters today?

When designing inverters, particular attention should be paid to ensuring that they function as efficiently as possible throughout their service life. This starts with a straightforward and quick installation process. One way in which Fronius achieves this for example is by supplying our devices with a separate wall bracket and connection compartment that is attached to the wall, and connected before the inverter itself is installed. This provides sufficient space for the device to be installed securely and quickly. Installation of the inverter itself simply involves plugging in the component. Furthermore, fully autonomous operation coupled with an indication and data communication system that is tailored to the individual user is absolutely essential.

Are DNOs becoming more relaxed about small generators coming onto national grids, and is this having any bearing on inverter design?

How “relaxed” the DNOs are, is very different from country to country and DNO to DNO. Typically there are different phases, and they correlate with the growth of PV. At the beginning PV was seen as small and unimportant. The next step was the finding that PV has an influence on the grid. In some cases this has lead to an opposing position of the DNOs, causing hurdles. But there has been a lot of constructive cooperation, which has led to much better understanding on both sides. There have also been important changes in the inverter design, with the aim of allowing a high PV penetration now and in the future.

What further progress would you like to see in sub-system and component design to enable inverters to become smaller/lighter/cheaper?

Currently we try to integrate more functions from subsystems into the inverters. As a example, we have integrated the DC-disconnector and the overvoltage protection into the inverter, to reduce the price for the whole system.

How efficient are your inverters and how efficient could inverters become with further evolution?

The maximum efficiency of our inverters is 95.9 % (Fronius IG Plus), and around 97,7 % for the IG TL product ranges. The efficiency of an inverter always depends on striking a balance between the components it uses and their cost. The greater the efficiency of the inverter, the more costly its components will be. However, the use of lower-loss components also has an effect on the size and weight of individual components. As far as efficiency is concerned, the manufacturer should aim to achieve an optimum balance between production costs and efficiency. At present, great store is being set by the “efficiency” value specified on the inverter's data sheet. Although this does serve as an indicator, the energy yield the device feeds into the mains in the particular system in question is more important.

It is also worth taking into account MPP adjustment efficiency, or the efficiency at different DC voltages; these may sometimes vary considerably from the maximum efficiency. For this reason our inverters have an active transformer switchover feature that ensures a consistently high degree of efficiency right across the input voltage range. The yield can also be further increased by features such as a master/slave concept, which enables greater partial-load efficiency.

Questions were answered by Martin Heidl and Hannes Heigl.

The case for micro-inverters was persuasively put to us by Henrik Raunkjaer, ceo of inverter producer Enecsys when your correspondent interviewed him at the recent EcoBuild exhibition in London. Enecsys, a technology spin-off from the Cambridge University Power Laboratory has developed a micro-inverter that meets the technical requirements for grid connection while being reliable, durable and affordable.

Raunkjaer emphasised that, for this type of solution to be viable, a micro-inverter should be as reliable and durable as the solar panel it serves - which may well be guaranteed for 25 years. Enecsys worked hard to engineer out failure-prone components and, at the same time, reduce overall component count. Thus, for instance, plastic film capacitors are used in place of more conventional electrolytic types often used as charge storage media in filter circuitry used to produce “clean” ac of a standard acceptable for the grid. Film capacitors do not dry out or contain liquids that can leak out and they have rated lives about four times that of electrolytics. They do, however, have less capacity, so circuits must be configured to require less charge storage. Optical couplers, another insufficiently reliable component used in some contemporary inverters, likewise had no place in the Enecsys micro-inverter design.

Component count was reduced, from nearly 350 to some 250, through efficient design and high circuit integration. Enecsys expects to reduce this still further, perhaps to as little as 100 components within three to five years.

The fact that, like the solar panels they are associated with, micro-inverters are out in the open in all weathers and must endure the heating effects of direct solar radiation, complicates the task of component selection. Enecsys micro-inverters are designed to withstand temperatures ranging between −40 and +85 deg C, along with daily thermal cycling in wet and dry conditions. The company claims there is no degradation of conversion efficiency at temperatures up to +85 °C.

The inverters' all-weather reliability is key in countering the claims of ‘centralists’ that maintenance would be a problem given the need to trace faulty units, access them in difficult locations and replace them.

Raunkjaer argues that failures will be rare and that a central monitoring system the company has developed will be able to identify faulty units remotely. Each module's real-time performance can be viewed separately, a capability that string inverters do not have. The monitor can not only pinpoint the exact location of any problem, but can also help indicate what maintenance action might be needed. A built-in wireless communication system connects to the Internet via a gateway unit so that information is available on line and can be viewed from any suitable networked computer.

Another advantage is that the failure of one panel or micro-inverter affects only the output of that part of the system. The rest continue to work normally. Also, it is not necessary to turn off the system in order to replace a micro-inverter; the maintainer simply unplugs one and plugs in its replacement.

Q&A: what the experts think – SMA

What are the main design and usage trends in inverters today?

The main trends in new PV inverters can be found in the areas of cost reduction, grid integration and the safety of system technology. By 2020, a further 50 percent reduction in system technology costs is required in order to compete with conventional power generation companies. Due to the high installed PV output compared to conventional power plants in some countries, an increasingly important factor is ideal grid integration and therefore also the transition from a “grid-following” to a “grid-providing” function. Higher safety standards are also required for PV plants, so that the plant is free of voltage for maintenance work or in the event of electric arcs.

Are DNOs becoming more relaxed about small generators coming onto national grids and is this having any bearing on inverter design?

Due to the considerable increase in PV plants - approximately 80 percent of which are connected to the distribution grid - the DNOs are more and more vocal in calling for a contribution from PV inverters towards voltage stability, to avoid expensive grid expansion. New types of inverters are capable of influencing the voltage at the connection point by feeding in reactive power, thereby minimising voltage increases. Modern inverters make a decisive contribution towards grid stability today. First of all they reduce their feed-in capacity according to the grid frequency when there is a surplus of energy in the grid. Secondly, in the event of a grid fault they do not immediately disconnect from the grid, rather they continue to support the voltage by feeding in reactive power.

What further progress would you like to see in sub-system and component design to enable inverters to become smaller/lighter/cheaper?

The key to smaller and more affordable inverters lies in a greater degree of electronic component integration, and reduced use of materials, especially for the enclosure and filter components. Among other things, new kinds of semiconductor components on a SiC basis will play an important role; with the same losses, it is possible to achieve a significant increase in switching frequency and thus smaller filter components.

How efficient are your inverters and how efficient could inverters become with further evolution?

SMA introduced the first commercially available inverter with a peak efficiency of more than 98 percent in 2006. At this time, this efficiency was more than 1 percent above the market standard. Today a peak efficiency of 98 percent represents the state-of-the-art for PV inverters. By using new semiconductor devices based on silicon carbide, a further reduction of losses can be achieved by a factor of two. Efficiencies of 99 percent are then feasible, but have not yet appeared on the market, due to the higher costs and unproven reliability of these new devices.

Questions were answered by Dr. Bernd Engel and Dr. Matthias Victor.

Henrik Raunkjaer asserts that because micro-inverters are wired in parallel rather than in series as central/string inverters normally are, they are safer. Series-connection of strings can, he points out, result in high dc voltages - up to several hundred volts in some cases - posing a danger to anyone working on a solar-clad roof and to the building fabric itself should an electrical arcing fault develop. Because no lethal high voltage dc is created, installers and maintainers do not need specialist skills and equipment. The inverters shut down automatically if the grid is disconnected or inverter temperature becomes too high.

Raunkjaer resists the argument that micro-inverters are generally less efficient than central/string inverters. While admitting that this can be true on the basis of a direct comparison of conversion efficiencies (Enecsys achieves a peak of about 94%), he asserts that arrays which rely on micro-inverters should harvest more energy over time. This is because when solar panels are connected in series, any degradation in the performance of one panel - whether due to a fault, dirt or shading of the panel - will affect the entire string of which it is a part. Because micro-inverters are connected in parallel, however, this domino effect is avoided.

A further boost is achieved because maximum power point tracking, the technique used to ensure that each solar panel is presented with the ideal electrical load whatever the incident light, is applied for each module rather than for a string of modules. This ensures maximum energy harvest even under partially shaded conditions. Overall, Raunkjaer claims, the total gain in energy harvested by using micro-inverters can be anything between 5 and 20%.

Enecsys is now marketing its 200, 240, 280 and 360W micro-inverters to solar integrators and distributors internationally. Raunkjaer believes that there is particular scope in Europe, where the distributed micro-inverter solution has yet to catch on as it has in the U.S.

He believes Enecsys to be the only company providing inverters that qualify as true global products since they can be configured in software for any grid standard. Certification has been achieved in a number of European and North American countries and is pending in others. Development will continue in Cambridge, UK, with support from Taipei in Taiwan, while manufacture takes place in China, Poland and may do so in other locations eventually.

In time, module makers may start to produce ac modules that combine micro-inverters and panels in single easy-to-install ‘plug and play’ products. First, though, they will want proof from field service that micro-inverters are reliable in the long term.

The fact that Enecsys inverters can already be offered with a 20-year warranty (provided that scheme designers also utilise the company's on-line monitoring system) suggests confidence that this proof may not be long in coming.

But whilst the micro inverter revolution looks set to spread, central and string inverters remain the mainstream, and as we will see progress is evident in this area too.

Part 2 of this article will look further at central and string inverters. It will appear in the May/June 2011 issue.

About:

George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.

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